Are you asking about the watch, or whether they're okay to buy from eBay?
I have a Citizen Cyber Aqualand NX dive computer watch which I bought in Jamaica for 500 USD. The watch is great. My only disappointment is regarding the readout. All the advertising bumf and even the box show illustrations of a metric readout, but mine shows depth in feet and temperature in fahrenheit. Believe it or not, this cannot be changed in any of the settings; the watch needs to go back to a Citizen service centre.
I'm afraid I can't speak for eBay - I'd be cautious though.
BB